1. Field of the Invention
The present invention relates to the manufacturing of single-pole components in vertical monolithic form. The following description more specifically relates to components of Schottky diode type made in vertical form in silicon substrates.
2. Discussion of the Related Art
FIG. 1 illustrates a conventional Schottky diode structure. Such a structure includes a semiconductor substrate 1, typically made of heavily-doped single-crystal silicon of a first conductivity type, generally N type. A cathode layer 2 covers substrate 1. It is N-type doped, but more lightly than substrate 1. A metal layer 3 forms a Schottky contact with N-type cathode 2.
The thickness of layer 2 is chosen to determine the reverse breakdown voltage of the Schottky diode.
FIG. 2 illustrates the variation of the electric field E across the thickness of the structure shown in FIG. 1, along an axis A–A′. For clarity, the different portions of curve 10 of FIG. 2 have been connected by dotted lines to the corresponding regions of FIG. 1.
In such a homogeneous structure, the field variation per thickness unit is proportional to the doping level. In other words, the field decreases all the faster as the doping is heavy. It thus very rapidly drops to a zero value in substrate 1. Since the breakdown voltage corresponds to the surface included between the axes and curve 10, to obtain a high breakdown voltage, the doping of layer 2 must be minimized and its thickness must be maximized.
In the manufacturing of single-pole components, opposite constraints have to be considered. Single-pole components, such as the diode shown in FIG. 1, must indeed have as small a resistance (Ron) as possible, while having as high a breakdown voltage as possible when reverse biased. Minimizing the on-state resistance of a single-pole component imposes minimizing the thickness of the most lightly doped layer (layer 2) and maximizing the doping of this layer.
To optimize the breakdown voltage without modifying resistance Ron, structures of the type of that in FIG. 3 have been provided. In FIG. 3, a vertical Schottky diode includes a single-crystal silicon semiconductor substrate 31, heavily doped of a first conductivity type, for example, type N, and coated with a layer 32. Layer 32 is formed of the same semiconductor material as substrate 31 and is of same doping type, but more lightly doped. Layer 32 is intended for forming the cathode of the Schottky diode. A metal layer 33 covers layer 32. The metal forming layer 33 is chosen to form a Schottky contact with N-type silicon 32.
Layer 32 includes very heavily-doped P-type silicon regions or “islands” 34. Islands 34 are distributed over at least one horizontal level (over two levels in the example of FIG. 3).
Islands 34 are separate and buried in layer 32. The islands 34 of different horizontal levels are substantially distributed on same vertical lines.
FIG. 4 illustrates the variation profile of electric field E across the thickness of a structure similar to that in FIG. 3. More specifically, the profile of FIG. 4 is observed along axis A–A′ of FIG. 3.
As appears from the comparison of FIGS. 2 and 4, the insertion of heavily-doped P-type “islands” 34 in the structure of FIG. 3 modifies the variation of field E per thickness unit. Since islands 34 are much more heavily doped than N-type layer 32, there are more negative charges created in islands 34 than there are positive charges in layer 2. The field thus increases back in each of the horizontal areas including islands 34. By setting the doping and the number of islands 34, the space charge area can be almost indefinitely widened. In reverse biasing, the cathode formed by layer 32 and islands 34 thus generally behaves as a quasi-intrinsic layer. In average, the electric field variation per thickness unit thus strongly decreases. Thus, for a given doping level of layer 32, the breakdown voltage is increased, as illustrated by the increase of the surface delimited by the axes and the curve of FIG. 4 as compared to the corresponding surface of FIG. 2.
Accordingly, the structure of FIG. 3 enables obtaining single-pole components of given breakdown voltage with a resistance Ron smaller than that of a conventional structure.
The practical implementation of such a structure with islands is described, for example, in German patent 19,815,907 issued on May 27, 1999, in patent applications DE 19,631,872 and WO99/26,296, and in French patent 2,361,750 issued on Mar. 10, 1978. These different documents provide obtaining a structure similar to that in FIG. 3 by performing implantations/diffusions during a growth epitaxy of layer 32.
The repeated interruptions of the epitaxial growth are a disadvantage of such an implementation. Indeed, thick layer 32 thus obtained has an irregular structure. Such structure irregularities alter the performances of the final component.